The preparation of polyurethane ureas from an isocyanate prepolymer and water at temperatures from 70.degree. to 100.degree. C. in a mixer operating at a low mixing intensity has been described in U.S. Pat. No. 4,049,632. This process, however, has only limited applicability because the prepolymers must be synthesized from difunctional polyesters with equivalent weights of from 480 to 540 and selected diisocyanates. In order to keep these prepolymers workable, i.e. stirrable, the NCO/OH ratio must not fall below a certain limit. On the other hand, if a certain NCO/OH ratio is exceeded in preparing the isocyanate prepolymer, the products obtained from the subsequent reaction with water are not homogeneous. The use of multishaft screw extruders is not mentioned.
Thermoplastic polyurethanes and their preparation from tolylene diisocyanate, polycaprolactone diol, a low molecular weight glycol and water has been described in U.S. Pat. No. 4,055,549. According to this patent specification, the products of the process are preferably produced discontinuously. When a continuous extruder process is employed, the reaction between tolylene diisocyanate and water is carried out before the components are introduced into the extruder to prevent evolution of carbon dioxide and foaming and the attendant buildup of pressure in the extruder. U.S. Pat. No. 4,055,549 teaches that the reaction between the isocyanate component and water is started at temperatures below 100.degree. C. Experience has shown that this low starting temperature does not result in a desirable distribution of polyurea hard segments and polyurethane soft segments in the resulting macro molecule.
The preparation of polyurethane or polyureas from polyisocyanates and polyols in the absence of water in multishaft screw extruders with kneading elements has been described in U.S. Pat. No. 3,963,679. However, there is no indication of how to produce these polyurethanes as polyureas in multishaft screw extruders with water without the problem of carbon dioxide evolution and the attendant pressure buildup.
It has now surprisingly been found that, contrary to the teaching of U.S. Pat. No. 4,055,549, it is possible to prepare polyurethane polyureas by the reaction of polyisocyanates with water in extruders, preferably multishaft screw extruders, without the polyurethane leaving the extruder having frothed.